Project Summary Overview: Diffusion MRI (dMRI) fiber tracking is the only imaging method with clinical potential to map fiber pathways in the human brain. Unfortunately, clinical application of dMRI fiber tracking is impeded by a lack of knowledge about the influence of histological features on accurate fiber tracking. Here, we propose to determine the influence of histological features on the dMRI signal through a comparison of dMRI fiber tracking and a new gold-standard 3D histology called CLARITY. Relevance: Improved deep brain stimulation targeting via dMRI fiber tracking has the potential to transform deep brain stimulation procedures by obviating the need for awake surgeries, reducing the number of brain penetrations (and thus reduce patient risk), eliminating imprecise, indirect-targeting methods and helping to expand the scope of neurological conditions that can be treated. Approach: To learn the basic relationships between histological features and dMRI tracking accuracy we propose to compare postmortem dMRI fiber tracking against direct optical observation of individual neurons using a gold-standard CLARITY in the same intact, fixed human brain tissue and then relate these results back to in vivo data. Summary: This proposal aims to define the basic relationships between the dMRI signal and histological features such that dMRI fiber tracking can be accurately applied and interpreted for neurosurgical targeting. Our research team includes world-class experts and inventors of high-resolution postmortem dMRI and CLARITY 3D histology. We have established a strong collaboration across the disparate fields of MRI physics, neuroradiology, neuropathology, neurosurgery, neurology and bioengineering (tissue clearing/optical imaging).